The present invention relates to novel carborane cholesterol analogs and their use in the treatment of tumor and cancers in humans, and in particular to the treatment of human brain tumors. Pharmaceutical compositions and methods of using these compositions in the treatment of tumors and cancer are other aspects of the present invention.
Cancer continues to be a major cause of death in humans. Conventional treatment such as surgery, radiation therapy and chemotherapy have been extremely successful in certain cases. In other instances, alternative forms of cancer therapy involving the use of boron have been investigated. For example, Boron Neutron Capture Therapy (xe2x80x9cBNCTxe2x80x9d) has been used to treat certain tumors for which conventional therapies have failed. Such therapies have helped treat Glioblastoma multiforme, a highly malignant, invasive form of brain cancer in Japan. BNCT is a two-step radiotherapy in which selective radioactive effect on tumor cells is achieved by targeting the tumor with non-radioactive 10B and subsequently exposing it to neutrons. The nuclear reaction between 10B and low energy neutrons creates high energy and particles, destroying those cells which contain 10B-containing agent.
BNCT is based on the nuclear reaction between thermal or epithermal neutrons, i.e.,neutrons having energies of less than 0.5 eV to about 30-100 KeV, and Boron-10 to yield tumor-destroying alpha particles and Lithium-7 nuclei. In BNCT of malignant brain tumors, the patient is injected with a boron compound highly enriched in Boron-10. If designed correctly, the boronated compound concentrates preferentially in the brain tumor, not in the healthy surrounding tissues. A beam of thermal neutrons irradiate the patient""s head and are then captured by the boron concentrated in the tumor. The tumor is thus irradiated with alpha particles whose range in tissue is about 10 xcexcm or the diameter of an average cell. A localized, specific reaction takes place whereby the tumor receives a large radiation dose in comparison to that received by the surrounding healthy tissue. Shrinkage of tumor tissue results.
Various boron-containing compounds have been investigated for the clinical use of BNCT. Boronated analogs of compounds such as amino acids, thiouracils, chlorpromazine, nucleosides, antibodies, etc. have been the focus of compound development.
The boron-containing compound Na2B12H11 SH (BSH) has been used in clinical trials. However, in-vitro experiments have shown that BSH does not stay in the tumor cell but is easily washed out. This may have accounted for the lack of success in some clinical trials.
Nucleosides have been proposed as alternative boron-containing compounds because such structures are proposed to be conserved by rapidly proliferating tumor cells and phosphorylated by cellular kinases to mononucleotides. These compounds can thus be localized to the tumor cell or can also be converted to the active precursors of nucleic acids, the di-and triphosphate forms.
BNCT-based nucleosides generally contain a single boron atom inserted into or linked or attached to the purine or pyrimidine base moiety. However, Soloway et al. , in U.S. Pat. No. 5,171,849, have described a process for preparing carboranyl uridine nucleoside compounds and their diethyl ether adducts which exhibit a ten-fold increase in boron content over prior boron-containing compounds. Such introduction of a carboranyl moiety into or on the carbohydrate portion is also believed to enhance lipophilicity for cellular entrapment in proliferating tumor cells due to action of its structure and possible retention by cellular kinases or by the incorporation into an oligonucleotide which would hybridize strongly with RNA or DNA sequences. Additionally, the diether adducts of carboranyl uridine have enhanced hydrophilicity to enable solvation in aqueous media and can also be used in BNCT.
Michiko et al., in U.S. Pat. No. 4,959,356, have described the usage of porphrin compounds in BNCT. Porphrins are naturally occurring tetrapyrrole compounds normally found in plants and animals. They combine with metallic ions to produce a metalloporphrin important in metabolism. For example, Hemin is an iron-containing porphrin essential to blood. These porphrin compounds have preferential affinity for neoplastic tissue and can accumulate in tumors as shown by fluorescence which is produced by UV light activation of porphrins or metalloporphyrin.
The porphrin compounds can be employed as vehicles for the transport of boron to malignant tumors, especially brain tumors. The blood-brain barrier in mammals excludes the uptake of boronated porphyrin compounds into normal tissue and allows the accumulation of porphyrin compounds in tumor cells. Thus, upon irradiation, significant damage will be done to the tumor while leaving the healthy tissue intact. Typically, the patient""s body is placed about 50 and 100 cm from the beam port.
Besides the type of boron carrier, methods for neutron-capture therapy have also been described. Russell et al., in U.S. Pat. No. 4,516,535, have noted that there has been a rapid attenuation of the thermal neutron flux which thus prevent effective treatment of deep-seated tumors. A large proportion of the neutrons never reach the tumor, but instead damage normal tissue. They have proposed the usage of an epithermal beam.
Thermal neutrons, which have energies of 0.5 eV or less, although more easily captured by the boron, are largely absorbed by the outer tissues before reaching the tumor. Epithermal (intermediate) neutrons with higher energies up to 100 KeV (preferred energy to 30 KeV), pass through outer layers of tissue and lose energy in the process. Then they are slowed to at or near the thermal energy range where they are subject to a high probability of capture by B10. Fast neutrons,on the other hand, possess high energies and are highly destructive to tissue. Thus, a preferred epithermal beam should have low fluxes of destructive fast neutrons and thermal neutrons. Additionally, epithermal neutrons are effective in destroying subsurface tumors.
Russell and colleagues have used a filtered neutron beam that includes aluminum, sulfur and argon filters that provide a neutron xe2x80x9ctransmissionxe2x80x9d filter. This filter selectively transmits epithermal neutrons, absorbs thermal and fast neutrons, and attenuates harmful gamma radiation. These beams have a neutron flux greater than 107 and preferably greater than about 5xc3x97107 neutrons/cm2 sec. A preferred neutron energy distribution has less than about 15% and preferably less than 5% of the neutrons having energies above 30 KeV and a gamma radiation dosage within a medically acceptable range. Adjusting the ratio of thermal to epithermal neutrons changes the beam penetration to the proper depth for specific tumor location.
Generally, the patient is injected with a suitable dose of Na2B12H11 SH (BSH) carried in a pharmaceutically acceptable medium between about 12 and about 24 hours prior to subjecting the patient to the filtered neutron beam. The patient is positioned between 50 and 100 cm from the beam port and in front of the beam of primarily epithermal neutrons.
In addition to the use of boron-containing compounds in BNCT, Kinder et al., in U.S. Pat. No. 4,963,655, have developed boronic acid analogues of amino acids that inhibit growth or colony formation in mammalian cells.
In cancer-related processes, increased protease enzyme activity ensues; such associated activity may be associated with the transformation of cells by viruses, chemicals, or other agents or with the metastatic potential of cancer cells. Data has suggested that protease inhibitors may prevent or reduce the incidence of transformation and reduce the metastatic potential. Kinder and collegues have described boronic acid analogues of amino acids which can act as protease inhibitors. In addition to the boronic acid analogues, they also showed a method of coupling boronic acid analogues of amino acids to other N-protected amino acid or N-protected dipeptides.
Antitumor agents generally work in the following manner. For example, they may be alkylating agents which bind covalently to DNA to form bifunctional lesions. Alternatively, they may function as antimetabolites which inhibit enzymes involved in DNA synthesis or assembly or serve as an analog substrate of DNA processes. Also, they may act as antibiotics which intercalate into the DNA helix or introducing strand breaks into DNA. Unlike these traditional mechanisms, the Kinder-described boron acid analogues function as protease inhibitors and mimic the transition state of serine-protease catalyzed peptide hydrolysis.
Normally, serine proteases hydrolyse peptide bonds by a nucleophillic attack of the serine OH in the active site of the enzyme on the carboxyl carbon of the substrate peptide bond. The nucleophilic attack forms a transient state. When boron is substituted for the carboxyl carbon at the reactive substrate site (e.g. where hydrolysis occurs). A stable-tight binding intermediate ensues.
Thus, these analogues inhibit the serine proteases elastase and chymotrypsin and also are cytotoxic against human and murine tumor cell in culture. However, the mechanism of cytotoxicity is still not fully understood. Further boron applications in cancer therapy are still yet to be defined.
It is an object of the present invention to provide novel carborane cholesterol compounds to be used in the treatment of tumors and cancer.
It is an additional object of the present invention to provide pharmaceutical compositions comprising novel carborane cholesterol compounds for use in the treatment of tumors and cancer, especially brain tumors.
It is another object of the present invention to provide methods for treating tumors and cancer which utilizes carborane cholesterol compounds according to the present invention.
It is a further object of the invention to provide a method for treating human brain tumors.